"Reek" was Roose Bolton's bastard son's (Ramsay Snow, now Ramsay Bolton) pet creature. Ramsay dressed Reek up in his clothing as they were being chased (by Karstarks) I believe, and Ramsay was captured and pretended to be Reek.

He revealed himself after sacking Winterfell and killing an army of fellow Northmen, and he kept Theon prisoner and tortured him, and told people that he was Reek (and everyone assumes Theon is dead).

Is the paperback ever coming out? I don't really feel like paying 20 for something I won't read again

Not for at least a year, probably more. Big name books take forever. For example: Still no paperback for The Girl Who Kicked the Hornets Nest, and actually none planned as of yet. (At least, nothing officially announced)